As is well known, punch presses are highly useful for producing various types of cutouts in sheet-like workpieces of metal, plastic, composite materials and the like. In recent years, numerically controlled punch presses have enabled the production of relatively complex cutouts by repetitive punch strokes in nibbling operations, because movement of a workpiece clamping mechanism may be controlled by an X-Y coordinate guidance system.
Moreover, with the advent of large turret assemblies in turret-type punch presses and with the later advent of automatic tool changer type punch presses, a fairly large amount of tooling may be provided within the press so that the operative punch and die may be readily changed. Thus, even complex configurations have been made possible through the optimization of computer numerically controlled systems in presses providing a multiplicity of interchangeable tools.
However, the configuration of cutouts that could be obtained by a punch press still has been dictated by the specific tooling available for operation without changing the tools in the turrets or in the tool changer carriers, and it has generally been necessary to change the tooling in the operative ram position in order to effect any modification in the cutout being produced. More recently in Hirata et al. U.S. Pat. No. 4,412,469 granted Nov. 1, 1983, it has been proposed to provide additional variation in the nature of the cutouts produced by a single set of tooling by providing a rotatable set of tooling in a turret-type punch press. More particularly, at two diametrically spaced positions in the turrets, there has been provided tooling which can be rotated by a pair of servomotors. As a result, rotation of the tooling may be coupled with the indexing of the workpiece to spaced positions to produce cutouts with the tooling rotated relative to the X and Y axes of movement of the workpiece so that the cutouts are at different angular relationships.
Even more recently, there has been introduced a tool changer type punch press in which the punch is rigidly coupled to the lower portion of the ram assembly so as to move integrally therewith and to provide a mechanism which effects rotation of the lower portion of the ram assembly and thereby the punch. Moreover, the die is firmly held in a die holder assembly which is rotated simultaneously with the ram assembly lower portion by the same drive mechanism. This punch press is illustrated and described in the copending application of Hans Klingel entitled "Punch Press With Rotary Ram And Method Of Operating Same", Ser. No. 661,399 filed Oct. 16, 1984.
The availability of such rotary action for the punch press tooling provided by these two rotary action presses has thus increased the versatility of a single set of tooling.
In nibbling or contouring operations, the punch tooling repetitively acts upon the workpiece as it is moved relative to the punching station to produce a series of overlapping cutouts which ultimately define a large contoured cutout, and this may have rectilinear and/or curvilinear edges. Generally, the tooling for such nibbling operations has utilized a relatively small diameter, generally cylindrical cutting portion on the punch and a cooperatively configured die, although polygonal cross section punches are also used. As is well known, the size of the punch that may be utilized is limited by the punching force generatable by the punch press, the thickness of the sheet material comprising the workpiece and the shear strength of the material comprising the workpiece. Moreover, the feed rate or rate at which the sheet materials is indexed to form the contoured edge on the workpiece will depend upon the amount of overlap of adjacent punch strokes which, in turn, is often dictated by the amount of roughness tolerable in the contoured edge.
As will be appreciated, nibbling of a straight line with the a circular punch will produce an edge defined by a series of arcs and the roughness will be dependent upon the feed rate per stroke as seen in FIG. 18a, where E represents the lineal advance of the workpiece or feed rate, D.sub.n =diameter of the nibbling tool, and R.sub.t =roughness or the width of the sector defined by the intersecting arcs of the adjacent punch cutouts.
The feed rate E for a straight edge is usually controlled to limit R.sub.t to 0.2-0.4 mm, determined in accordance with the following formula: ##EQU1## If the allowable roughness is greater and the diameter of the punch can be greater, then the feed rate E can be increased to produce the nibbled cutout faster.
Similarly, if a curved edge is to be nibbled, the feed rate is also dependent upon the amount of roughness as seen in FIG. 18b, where E=represents the lineal advance of the workpiece between strokes, R.sub.z =the radius of the punch, R.sub.s =the radius of the curve for the cutout, and R.sub.t =roughness or the length of the sector defined by the intersecting arcs of the adjacent cutouts. The comparable formula is as follows: ##EQU2##
By use of a punch with an arcuate cutting face having the same radius as that of the desired curve for the cutout, the roughness produced by the nibbling can be eliminated. However, in almost all instances this would require a circular punch with a very large diameter and in turn requiring more power than the press has available to punch through the workpiece.
The force to cut through the workpiece with a circular punch is, for a single stroke cutout, determined in accordance with the formula: EQU F=D.sub.s .multidot..pi..multidot.S.multidot.T.sub.s
where D.sub.s is the diameter of the punch, S is the thickness of the sheet material, and T.sub.s is the shear strength of the sheet material. For the force required using a rectangular punch, the formula is: EQU F=(2L+2W).multidot.S.multidot.T.sub.s
where L and W are the length and width of the punch cross section. It can be seen that the force required can rapidly reach and exceed the capacity of the press as thicker materials are employed.
A further factor that must be considered in punch design is that the width of the punch must be at least as great as, and preferably greater than, the thickness of the sheet material. If not, the punch is subject to breakage. Moreover, polygonal punches wear more rapidly in cutting sheet material than does a tool of circular cross section, and this is particularly significant as thicker sheet material is being punched.
In the copending application of Rudi Kuppinger, entitled "Method and Apparatus for Nibbling Cutouts by Rotation of Tooling with Cutting Surfaces of Different Contours and Tooling Therefor", Ser. No. 661,381 filed Oct. 16, 1984, there is disclosed an improved punch press apparatus and method for nibbling large contoured cutouts in workpieces using tooling wherein the punch cutting portion provides at least two cutting surfaces of different contours. As the workpiece is indexed, the tooling is rotated to provide an angular orientation of the desired cutting surfaces which closely conforms to the contour desired for that portion of the cutout. The tooling specifically illustrated is of elongated curvilinear cross section with a length L and a transverse width W which is less than 1/2 L. The arcuate faces along the longitudinal axes are generally determined by different radii and the preferred cross section is concavo-convex with the end portions blending into convex curved surfaces. The elongated curved cutting surfaces provided by this elongated punch tooling allows close approximation of the curvature of the desired contoured edge to eliminate roughness, and fewer strokes and a higher feed rate to produce the cutout. As the workpiece is being indexed, the punch tooling is rotated so as to closely conform to the desired contour.
This solution to the problem of contouring cutouts in nibbling operations has provided significant benefits. However, although it is beneficially employed to reduce roughness in rectilinear contours by reason of the large radius arcs that may be provided on the tooling, there has remained the problem of further reducing roughness along rectilinear contoured edges and in making clean cuts at corners. Although substitution of other sets of tooling for these purposes may be employed, this increases the time for the nibbling operation and increases the cost.
It is an object of the present invention to provide a novel punching method for nibbling sheet material workpieces to form contoured cutouts with both curvilinear and rectilinear surfaces and at a relatively rapid rate with one set, or a limited number of sets, of tooling.
It is also an object to provide such a method in which the tooling will generate both curvilinear and rectilinear contoured edges which are smooth or have little roughness.
Another object is to provide novel punch press assembly using such tooling for nibbling cutouts with straight and curved surface portions and including means for effecting controlled, rapid and arcuate rotation of the tooling and indexing of the workpiece along X and Y axes.
A further object is to provide novel punch tooling for rapidly nibbling contoured cutouts with both rectilinear and curvilinear portions in a workpiece when used in a punch press which will effect rotation of such tooling and precise indexing of the workpiece.